Many Monarch butterflies hatched in the Oamaru Gardens. Where have they gone?

George Gibbs, an entomologist at Victoria University of Wellington, responded.

The straightforward answer is that they normally do not persist south of Marlborough
or Nelson in the South Island. This means that although individuals can roam further
south during warm summers, and they may breed a summer generation or two, the
climate is marginal for them and they are unlikely to overwinter there. Thus in
Canterbury and Otago the monarch does not normally succeed as a viable breeding
population. I am suggesting that in Oamaru perhaps you do not see monarchs every
year. When the spring-summer is favourable (or when people bring caterpillars in
from further north!) you will see them around the gardens but normally they will
perish at the end of the season. This is what I mean by marginal for them. Some
favourable years they do well, but then several years may pass without seeing any.

There are some problems with this simple answer. First, in North America where they
originated from, they are famous for their long distance migrations from Canada to
Mexico. Migrations have not been recorded in New Zealand but may eventually
develop so that butterflies bred in the south will fly north for the winter. Either way,
they will disappear from Oamaru gardens in autumn. Second, the assisted movement
of monarch butterflies by people will mess up their natural behaviour. I am aware that
some enthusiasts in Dunedin breed up summer generations for release. These efforts
will give you summer monarchs but are unlikely to establish a permanent population.

Monarch butterflies survive for several months as adults. Thus the individuals you see
in April may have been caterpillars back in December and freshly hatched butterflies
in autumn will survive through until October-November (in northern regions where
they can overwinter). The caterpillars do not survive cold winter weather and adults
are unable to lay eggs between about April and October.